A kelp with integrity: Macrocystis pyrifera prioritises tissue maintenance in response to nitrogen fertilisation
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Our understanding of the response of vascular, terrestrial plants to nitrogen (N) addition is advanced and provides the foundation for modern agriculture. In comparison, information on responses of marine macroalgae to increased nitrogen is far less developed. We investigated how in situ pulses of nitrate (NO3 −) affected the growth and N physiology of Macrocystis pyrifera by adding N using potassium nitrate dissolution blocks during a period of low seawater N concentration. Multiple parameters (e.g. growth, pigments, soluble NO3 −) were measured in distinct tissues throughout entire fronds (apical meristem, stipe, adult blade, mature blade, sporophyll, and holdfast). Unexpectedly, N fertilisation did not enhance elongation rates within the frond, but instead thickness (biomass per unit area) increased in adult blades. Increased blade thickness may have enhanced tissue integrity as fertilised kelp had lower rates of blade erosion. Tissue chemistry also responded to enrichment; pigmentation, soluble NO3 −, and % N were higher throughout fertilised fronds. Labelled 15N traced N uptake and translocation from N sources in the kelp canopy to sinks in the holdfast, 10 m below. This is the first evidence of long-distance (>1 m) transport of N in macroalgae. Patterns in physiological parameters suggest that M. pyrifera displays functional differentiation between canopy and basal tissues that may aid in nutrient-tolerance strategies, similar to those seen in higher plants and unlike those seen in more simple algae (i.e. non-kelps). This study highlights how little we know about N additions and N-use strategies within kelp compared to the wealth of literature available for higher plants.
KeywordsGrowth Nutrient limitation Translocation Stable isotope Differentiation
We thank the staff and students of the University of Otago Marine Science Department, the Portobello Marine Laboratory, and Liina Pajusalu for providing assistance in the field. David Duggins provided useful comments during the final stages of preparation of this manuscript, and input from reviewers on its first submission improved the clarity. This work was funded by a University of Otago International Postgraduate Scholarship and postgraduate research funding to T. A. S., and was supplemented by Departmental Funding from C. D. H.
Author contribution statement
T. A. S. and C. D. H. conceived the experiment. T. A. S. was responsible for the experimental design, performed the lab and field experiments and did the chemical and data analyses. T. A. S. wrote the manuscript with editorial advice from C. D. H.
Compliance with ethical standards
This research was not funded by any body independent of the University of Otago. It was funded through the Marine Science Department at the University of Otago via postgraduate research funding to T. A. S. and personal research funds given to C. D. H. as part of his professorship.
Conflict of interest
The authors declare no conflict of interest.
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